For an SD card to be usable by any kind of system, it needs to have a file system format on it. If you formatted the card on a computer, it may be in a format that the handset cannot read such as NTFS or HFS+. Generally, BBOS would format the card to a FAT variant that any device can also read. Move any data you loaded on the card off, allow the handset to format it and then transfer the data back.